A fixed type constant velocity universal joint is an example of a constant velocity universal joint used as means for transmitting torque from an engine of an automobile to wheels at constant velocity. The fixed type constant velocity universal joint connects two shafts on a driving side and a driven side and is provided with a structure allowing constant-velocity transmission of rotational torque even when the two shafts assume an operating angle. Generally, as an example of the widely-known fixed type constant velocity universal joint described above, there may be given one adopting a birfield type (BJ) or an undercut free type (UJ).
For example, as illustrated in FIGS. 28 and 29, the fixed type constant velocity universal joint of the BJ type includes an outer joint member 3 as an outer member having an inner spherical surface 1 in which multiple track grooves 2 are equiangularly formed along an axial direction, an inner joint member 6 as an inner member having an outer spherical surface 4 in which multiple track grooves 5 paired with the track grooves 2 of the outer joint member 3 are equiangularly formed in the axial direction, multiple balls 7 interposed between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6 so as to transmit torque, and a cage 8 interposed between the inner spherical surface 1 of the outer joint member 3 and the outer spherical surface 4 of the inner joint member 6 so as to retain the balls 7. In the cage 8, there are circumferentially arranged multiple pockets 9 for accommodating the balls 7.
A center curvature O2 of the track grooves 5 of the inner joint member 6 and a center curvature O1 of the track grooves 2 of the outer joint member 3 are offset (track offset) with respect to a joint center O by equal distances F, F in opposite directions along the axial direction, respectively.
Further, as illustrated in FIGS. 30 and 31, the fixed type constant velocity universal joint of the UJ type includes an outer joint member 3 as an outer member having an inner spherical surface 1 in which multiple track grooves 2 are equiangularly formed along an axial direction, an inner joint member 6 as an inner member having an outer spherical surface 4 in which multiple track grooves 5 paired with the track grooves 2 of the outer joint member 3 are equiangularly formed in the axial direction, multiple balls 7 interposed between the track grooves 2 of the outer joint member 3 and the track grooves 5 of the inner joint member 6 so as to transmit torque, and a cage 8 interposed between the inner spherical surface 1 of the outer joint member 3 and the outer spherical surface 4 of the inner joint member 6 so as to retain the balls 7. In the cage 8, there are circumferentially arranged multiple pockets 9 for accommodating the balls 7.
Each of the track grooves 2 of the outer joint member 3 is constituted by an inner side track groove 2a having a track groove bottom constituting an circular-arc portion and by an opening side track groove 2b having a track groove bottom constituting a straight portion parallel to an axial line of the outer joint member. In the inner side track groove 2a, the center curvature O1 thereof is shifted from the joint center O to an opening side of the outer joint member 3 in an axial direction. Further, each of the track grooves 5 of the inner joint member 6 is constituted by an inner side track groove 5a having a track groove bottom constituting a straight portion parallel to an axial line of the inner joint member and by an opening side track groove 5b having a track groove bottom constituting an circular-arc portion. The center curvature O2 of the opening side track groove 5b is provided while being separated in the axial direction from the joint center O by equal distances k to an inner side opposite to the center curvature O1 of the inner side track groove 2a of the outer joint member 3.
Further, in an outer spherical surface 8a of the cage, a center curvature O3 thereof is shifted from the joint center O to an opening side of the cage 8 in an axial direction, and in an inner spherical surface 8b of the cage, a center curvature O4 thereof is provided while being separated in the axial direction from the joint center O by equal distances k1 to the inner side opposite to the center curvature O3 of the outer spherical surface 8a of the cage. In the conventional fixed type constant velocity universal joint of this type, track offset amounts of the inner and outer joint members are set to be large, and an offset amount of the cage is set to be small. Further, pitch angles of two of the balls 7, which are adjacent to each other, are 60 degrees. That is, the balls 7 include six balls arranged at pitches of 60 degrees along a circumferential direction.
In recent years, in order to achieve compactification, there has been provided a fixed type constant velocity universal joint for an automobile, which is provided with eight balls as illustrated in FIG. 29 (Patent Document 1). In this case, a ratio between a pitch circle diameter of balls and a diameter of the balls is set to a predetermined value. In this context, the pitch circle diameter of the balls is represented by 2×PCR. Further, PCR represents a length of a line connecting the center of the track grooves of the outer joint member or the center of the track grooves of the inner joint member and the centers of the balls.
That is, in comparison with the fixed type constant velocity universal joint provided with six balls, in the fixed type constant velocity universal joint for an automobile, which is described in Patent Document 1, the balls are downsized and the number of the balls is increased. In this case, in accordance with downsizing of the balls, a load level of torque capable of being born by each of the balls is lowered. In order to compensate this, the number of balls is increased from six to eight.
However, when the number of the balls is to be increased for the purpose of further compactification of the outer diameter of the constant velocity universal joint, pillar widths (circumferential lengths) between the pockets of the cage are reduced. The rigidity of the pillar portions of the cage is important in securing joint strength at the time of a high operating angle.
Thus, regarding the constant velocity universal joint provided with eight balls, when the thickness of the pillar portions is decreased more than that according to the present setting, it is impossible to sufficiently secure strength thereof at the time of a high operating angle. Therefore, it is difficult to aim at further compactification.
In this context, in recent years, regarding the fixed type constant velocity universal joint provided with six balls, there has been proposed one in which downsizing thereof is achieved and torque load capacity is increased. That is, balls as large as possible are planned to be arranged on a PCD as small as possible. However, when large balls are used, pillars (window pillar) between the pockets of the cage are narrowed so that rigidity of the cage is decreased. In particular, breakage of the cage is liable to be caused by twisting torque load at the time of a high angle, which leads to deterioration in strength of the constant velocity universal joint.
Further, when the track offset amounts of the inner and outer joint members are large and the offset amount of the cage is small, the depth of the track grooves on the inner side of the outer joint member is decreased and the torque load capacity at the time of a high angle is decreased. That is, against the torque load at the time of a high angle, the balls are liable to climb onto the track edges, and hence excessive stress is generated on the edge portions. Therefore, damages are caused by chips of the edge portions, and a locking phenomenon with respect to the cage occurs in accordance with generation of plastic deformation. Those damages and locking phenomenon lead to deterioration in operability, decrease in lifespan, and the breakage of the cage. Further, in the inner joint member also, the depth of the track on the inner side is decreased, and there are involved disadvantages as those in the outer joint member. Thus, conventional attempts have been made to enhance joint strength and durability in a region of a high angle.
In this context, there has been conventionally provided a constant velocity joint in which circumferential dimensions of pillar portions existing between pockets adjacent to each other in a circumferential direction are increased for securing durability of the cage, whereby the entire durability of the constant velocity joint is enhanced (Patent Document 2). That is, in the constant velocity joint described in Patent Document 2, multiple balls are accommodated in one pocket, whereby intervals between the balls within the same pocket are reduced so as to increase the circumferential dimension of the pillar portions existing between the pockets adjacent to each other in the circumferential direction, and seven numbers or more of balls are assumed.
Further, there has been conventionally provided a universal joint in which two balls are accommodated in each pocket of a cage so as to suppress periodical fluctuations in moment during rotation (Patent Document 3). That is, in the universal joint described in Patent Document 3, all the pockets of the cage are formed as long windows with large circumferential intervals, and the circumferential length of one window pillar is increased.